What is the main function of a file extension?

Question

to indicate how much space the file takes up
to allow the file to be extended to new uses
to indicate which version of the file it is
to indicate which program is needed to open it

Answer 1

The main function of a file extension is to indicate which program is needed to open it. File extensions help operating systems determine the appropriate application to use for opening a file, based on the type of content it contains.